While I've had a Twitter account for a couple of years, I have only recently begun to utilize it. And while I'm definitely not an expert in Twitter strategy, I am a user and therefore I know what practices annoy the bajeebers out of me.
Twitter is like a mini blog and should generally follow the same guidelines as a regular blog. So in a regular blog, at least every one that I've ever read, each entry is unique. Yes, there may be recurring themes and topics but no one ever reposts a single entry multiple times. By and large, Twitter should be the same. However, some companies I've noticed like to tweet the same thing over and over. Perhaps they have one important message that they're trying to share like, "We're hiring" but they also like to share other useful things like, "Hey check out this link". And so after each time they post a "Hey check out this link" tweet, they repost the "We're hiring" tweet.
To me this practice is Twitter SPAM. It's unprofessional. It's rude to your followers. It encourages your followers not to pay attention to your tweets - which is the absolute last think you want to happen. And if you happen to be a company that claims to be a social media expert, it can be bad for business.
Consider the similarities and contrasts between two types of guns: the flare gun and handgun. Both are used to launch projectiles. Both are used to convey a message. A flare gun, however, will only shoot off one round at a time while a hand gun can fire many rounds in short succession. You run towards a flare gun but away from a handgun.
Twitter, used effectively, is like a flare gun because you want people to come to you. Therefore, you carefully plan your tweets and each contains a unique message. On the other hand, Twitter, used as I described above, is like a handgun. There's no strategy; you're just blasting away and people are going to run because it's the same thing over and over.
Of course, I think there's a third type of Twitter gun too, the machine gun. These are the companies that just don't stop sending out tweets. Seriously? While you are a news service, you're going to send out a tweet every three minutes when you post a new article? UN-SUB-SCRIBE! Maybe some people don't mind that but to me it just clogs up the feed and makes the useful information impossible to glean.
So if you run a company with an active Twitter strategy, think about how you're affecting your followers. Plan your tweets and don't inundate your followers with repetitive information.